1. Field of the Invention
The present invention relates to a harmonic mixer and more particularly, to a harmonic mixer configured from complementary logic 3-terminal devices which provide improved conversion gain, port impedance and isolation relative to known harmonic mixers.
2. Description of the Prior Art
Harmonic mixer-based frequency converters are useful in receiver applications and offer: lower phase noise through the use of a lower frequency voltage controlled oscillator (VCO); lower noise figure by suppressing the local oscillator (LO) noise sidebands; and reduction of baseband interference due to excellent LO radio frequency (RF) isolation and suppression of the LO self-mixed baseband products. In some cases, where the VCO frequency capability is limited by the device technology, a subharmonically pumped harmonic mixer approach provides a practical means of generating a high frequency LO source. Moreover, in cases where very low phase noise performance is required, a lower frequency VCO may be used and either frequency multiplied up to the LO source frequency or directly fed into a harmonic mixer as shown in FIG. 1. In particular, FIG. 1 illustrates that the subharmonically pumped mixer approach can result in superior phase noise performance due to the use of a lower frequency and lower phase noise (higher Q-resonator) VCO design compared to a fundamental frequency VCO employing a lower Q-resonator at a high frequency.
Conventional harmonic mixers are configured with anti-parallel diodes as the mixing elements and possess the characteristic of generating an effective conductance, g, which consists of a dc component and even harmonics of the LO frequency. FIG. 2 illustrates an anti-parallel mixing device in a subharmonic mixer application. The nonlinear characteristics of the anti-parallel diodes enable the generation of the subharmonically pumped LO mixing products ωIF=m·ωLO±n·ωRF where m+n=1,3,5 . . . . A detailed description of operation of an anti-parallel harmonic mixer is described in detail in Cohen et al, “Harmonic Mixing With an Antiparallel Diode Pair”, IEEE Transactions on Microwave Theory and Techniques, vol. MTT-23, pp. 667-673, August 1975, hereby incorporated by reference.
However, there are many practical design drawbacks to the use of the harmonic diode mixer. One such drawback is the bilateral nature of the LO, RF, and intermediate (IF) ports of the conventional anti-parallel diode harmonic mixer shown in FIG. 2. Since the anti-parallel diodes are strictly two terminal devices and do not provide unilateral characteristics, it does not provide signal directivity which typically is characteristic of 3-terminal solid-state devices, such as bipolar transistors. The consequence is that the isolation between the output IF port and the input LO and RF ports is strictly determined by the filter and passive networks in series with each port. The matching on one port will be dependent on the out of band impedances of the filters and matching networks of the other ports and visa versa. In particular, spurious mixer products can be produced by the back reflection of the IF mixer output products which do not see a matched 50Ω since they are out of the IF band. This can result in poorer mixer conversion efficiency. Another drawback is that the anti-parallel diode mixing device is typically a low dynamic impedance device that is very dependent on the LO drive level and can present sensitivity problems. This low impedance is seen by each of the RF, LO, and IF ports which sometimes results in less than optimum performance when the LO operating power is varied from its optimally matched LO power condition. Yet, the major disadvantage is that the anti-parallel diodes cannot produce conversion gain since it is a 2-terminal device with an output resistance equal to the reciprocal of its conductance whereas, the employment of a 3-terminal device, such as a bipolar transistor offers the ability to provide gain due to its unilateral characteristics which enables it to have a high output resistance, much higher than the reciprocal of its tranconductance. These and other shortcomings can compromise the overall performance and operation of the anti-parallel diode based subharmonically pumped mixer. Thus, there is a need for a harmonic mixer with improved conversion gain, port impedance and isolation relative to known harmonic mixers.